1. Field of the Invention
The present invention relates to a method for preparing a pure water. Particularly, the present invention relates to a method for preparing a pure water of a very high purity, so to speak an ultrapure water, suitable for use in electronic industries, for example, as a cleaning water used in the preparation of ultra LSI.
2. Discussion of Background
There are known many methods for preparing a pure water. For instance, a typical method comprises subjecting a basic water to be cleaned to direct filtration to remove suspended substances and treating the resultant water with an ion exchange resin to remove the majority of ionic substances present therein, thus producing a deionized water. The deionized water is then subjected to an ultraviolet ray irradiation step for decomposing organic substances present therein, a step for removing dissolved gases under reduced pressure, a step for removing dissolved ionic substances with a mixture system of a strong acidic cation exchange resin and a strong basic anion exchange resin, a step for removing dissolved substances by a reverse osmosis membrane and the like, thereby producing a pure water. The pure water thus obtained has water qualities preferably of a specific resistance of at least 18M.OMEGA..multidot.cm and a total organic carbon content of not higher than 10 ppb.
The pure water thus obtained is further purified to obtain an ultrapure water. A typical example of the purification is conducted by irradiating the pure water with ultraviolet ray of a short wavelength to decompose the organic matters contained therein into carbonic acid or carboxylic acid. The pure water thus treated with the ultraviolet ray is then treated with a strong basic anion exchange resin, and is further treated with a mixed bed system of a strong acidic cation exchange resin and a strong basic anion exchange resin to remove ionic substances formed by the ultraviolet ray irradiation. The resultant water is finally treated with an ultrafiltration membrane to remove suspended substances such as colloidal substances present in a very small amount, thereby producing an ultrapure water.
In the preparation of a pure water of a high purity or an ultrapure water, the total organic carbon content of which is not higher than 10 ppb, the water quality of the finally obtained pure water or ultrapure water is largely affected by impurities eluted from the materials constituting the purification apparatuses. Particularly, since an ion exchange resin comprises particles of a small particle size, the ion exchange resin has a very large surface area in comparison with a resin volume, and therefore the pollution by impurities eluted from the ion exchange resin to the finally obtained pure water or ultrapure water is not negligible. That is, when water to be purified passes through the ion exchange resin bed, the impurities in the water are adsorbed and removed by the ion exchange resin, and at the same time a small amount of impurities are eluted from the ion exchange resin. Accordingly, the purity of the water does not reach higher than a certain level of standard. In order to reduce the impurities eluted from the ion exchange resin, water is previously flown through the ion exchange resin column for a long period of time to remove elutable substances in the resin. However, this method consumes a part of water product in the production process, and consequently the efficiency of the process is lowered. Accordingly, an ion exchange resin which does not elute impurities so much during passing water is desired.
Generally, as compared with a sulfonic acid type cation exchange resin, it is difficult to reduce elutable substances from a quaternary ammonium type anion exchange resin simply by passing water, and therefore a quaternary ammonium type anion exchange resin which does not contain elutable substances so much is particularly desired.
The present inventors have studied substances eluted when a regenerated form ion exchange resin comprising a copolymer of a monovinyl aromatic compound and a polyvinyl compound as the matrix is shaken in a hot water at 50.degree. C., and have found that among the eluted substances, an organic compound having a molecular weight of less than 3,000 can be easily adsorbed by an ion exchange resin having a reverse charge, i.e. the organic compound eluted from a basic anion exchange resin can be easily adsorbed by a sulfonic acid type acidic cation exchange resin, but an organic compound having a molecular weight of at least 3,000 can hardly be adsorbed even by an ion exchange resin having a reverse charge, and that this phenomena is particularly remarkable with regard to the substances eluted from a basic anion exchange resin. Accordingly, when water to be purified is flown through a mixed bed ion exchange column comprising a cation exchange resin and an anion exchange resin containing substantially no elutable substances having a molecular weight of at least 3,000, the amount of the eluted substances contained in an ultrapure water obtained is very small. In the production of an ultrapure water, it is general to finally pass water to be purified through a cartridge type mixed bed ion exchange column, and accordingly this discovery means that an ion exchange resin, particularly a basic anion exchange resin, for preparing an ultrapure water is not necessarily washed to completely remove elutable substances, but has only to be washed until elutable substances having a molecular weight of at least 3,000 are substantially removed. Also, this washing step is preferably conducted so as to efficiently remove elutable substances having a molecular weight of at least 3,000.